[SelectionDAG] Expand nnan FMINNUM/FMAXNUM to select sequence

InstCombine may synthesize FMINNUM/FMAXNUM nodes from fcmp+select
sequences (where the fcmp is marked nnan).  Currently, if the
target does not otherwise handle these nodes, they'll get expanded
to libcalls to fmin/fmax.  However, these functions may reside in
libm, which may introduce a library dependency that was not originally
present in the source code, potentially resulting in link failures.

To fix this problem, add code to TargetLowering::expandFMINNUM_FMAXNUM
to expand FMINNUM/FMAXNUM to a compare+select sequence instead of the
libcall. This is done only if the node is marked as "nnan"; in this case,
the expansion to compare+select is always correct. This also suffices to
catch all cases where FMINNUM/FMAXNUM was synthesized as above.

Differential Revision: https://reviews.llvm.org/D70965